I used to work in a opthalmology devices manufacturing company, the product i was working on was fundus camera. During the planning phase we were working with a doctor who used to give us inputs for each phase of the device planning, he used to test the device every time during the device cycle on the subjects and used to give his feedback on it. His feedback was taken positively and was taken into consideration while in the implementation phase.
When the device was in the market there were many requests for changes in the device which we took positively as ultimately we want to give a good product to the customer and since he/she is paying it they have all the right to give their feedback.
Our device required dilation of eye for taking the image and the customer had asked us to have the device without dilation which was very unreasonable as it was a budget device and the image quality would be lower if we did that.